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Image Quiz: A cat with a large tail mass

This two-year-old spayed female cat was presented for evaluation of a large (4- x 4.5- x 5.4-cm) tail mass. On palpation, the mass felt turgid and hot, appeared to be causing pain, and had serosanguineous discharge (see photo above). Radiographs revealed a large soft tissue swelling with lysis of caudal coccygeal vertebral bodies (see photo below).

An excisional biopsy, which was palliative, provided a large sample for histopathologic examination. Representative immunohistochemistry fields from the sample are shown below.